The present invention relates to braking circuits for universal motors.
Universal motors are fractional horsepower AC motors specially designed for use on either AC or DC power. These motors have a commutator similar to those used in DC motors, a rotor winding constructed in a manner compatible for AC or DC use, and at least one field winding in series with the rotor winding. Universal motors are commonly used in household appliances such as vacuum cleaners, hand drills, saws, blenders, etc. These motors are constructed so they can be used in areas where only DC power is available in addition to areas where AC power is available.
When a universal motor is turned off by removing power, the motor continues to spin due to the inertia of the rotor. This continued spinning can be undesirable and even hazardous in some applications, such as a power saw where it would be desirable to have the blade stop once the saw is switched off. A braking force can be applied to a saw blade by a mechanical brake pad which is activated by the removal of power.
Dynamic braking is used for some DC motors using a shunt field winding as shown in FIG. 1A. A shunt field winding 50 is in parallel with a rotor 52 rather than being in series with it. In this arrangement, a switch 54 can be used to disconnect shunt winding 50 in parallel with the rotor and substitute a resistance 56 in parallel with the rotor when power is removed to stop the rotor. The rotor, which continues to spin due to inertia, acts as an electrical generator when power is removed. By switching this resistance in parallel with the rotor, an electric load is applied to the generator, causing the motor to function as a loaded generator and develop a retarding torque that will rapidly stop the motor.